Maxygen Announces Hold on MAXY-alpha
Development Program

REDWOOD CITY, Calif., Sept. 21 /PRNewswire-FirstCall/ -- Roche has advised Maxygen, Inc. (Nasdaq: MAXY) that it has voluntarily placed a hold on further clinical development of MAXY-alpha, also known as R7025. MAXY-alpha is a novel interferon-alpha for the treatment of Hepatitis C and Hepatitis B virus infections and is licensed to Roche.

Preliminary observations from a Phase I trial indicate that an unexpected reduction of the pharmacodynamic and pharmacokinetic effects of MAXY-alpha occurred in the majority of subjects who received two doses of MAXY-alpha. In addition, antibodies binding to MAXY-alpha were identified in some subjects. Roche has initiated additional investigational studies in order to assess these results.

"We are all surprised by these unexpected findings," said Russell Howard, Maxygen's chief executive officer. "We don't yet know how this will impact the future timing or advancement of the program. Roche has now started additional work to assess the meaning and significance of these results. We will provide an update once all relevant information is collected and evaluated."

About the Maxygen and Roche Agreement

Maxygen and Roche entered into an agreement in 2003 to develop novel interferon alpha and beta products for a wide range of indications. Roche licensed from Maxygen worldwide commercialization rights to specific novel interferon product candidates for the treatment of Hepatitis C and B virus infections. Maxygen received an initial payment, full research and development funding for work done by Maxygen in the first two years of the collaboration, and milestone payments for the advancement of the MAXY-alpha product candidate. In addition, Maxygen is eligible to receive milestone payments and royalties based on any product sales.

About Maxygen

Maxygen is a biopharmaceutical company focused on developing improved versions of protein drugs. We look for opportunities where our proprietary protein modification technologies can address significant therapeutic needs. For more information about Maxygen's products and technologies, visit http://www.maxygen.com.